Conventionally, a discharge lamp has been used that employs a fluorescent substance as a light source. Among the discharge lamps, a discharge lamp of hot-cathode type has been used as a backlight of a liquid crystal display (LCD) as well as for lighting because discharge lamp of this type has a high level of luminous efficiency and hence a high degree of luminance.
The discharge lamp of hot-cathode type has a configuration in which its glass tube is equipped with an electrode at each of its two opposed ends, a rare gas such as argon and mercury are enclosed in an internal space of the glass tube, and a fluorescent substance is coated into an interior of the glass tube.
FIG. 1 is a cross-sectional view of a configuration of a conventional discharge lamp of hot-cathode type. A discharge lamp 51 is equipped with an electrode 53 at each of two opposed ends of its glass tube 52. A rare gas such as argon, and mercury are enclosed in an internal space of the glass tube 52, and a fluorescent substance 52a is coated into a predetermined region in an interior of the glass tube 52.
The electrode 53 includes a heater 54 having a coil portion 54a. To the heater 54, an electron emission material 53a such as barium oxide is applied. The heater 54 is stretched with tension between two lead-in wires 55 inserted through an end of the glass tube 52 and held in position thereby. Therefore, in the electrode 53, the coil portion 54a of the heater 54 is arranged sideways so as to be perpendicular to a tube axis of the glass tube 52.
The light emission principle of the discharge lamp 51 of hot-cathode type will be explained as follows: when a voltage is applied between the two electrodes 53 at a high frequency in a condition where, by means of energizing these electrodes 53 the heater 54 heats the electron emission material 53a, the electron emission material 53a emits electrons to cause to be generated arc discharge between the electrodes 53.
The electrons emitted from the electron emission material 53a and then accelerated collide with mercury atoms so as to excite them. The mercury atoms thus excited emit ultraviolet light. This ultraviolet light is converted into visible light by the fluorescent substance 52a, thereby reducing the discharge lamp 51 luminiferous.
Conventional discharge lamps of hot-cathode type face a problem such that so-called ion sputtering in which any ions generated during discharge collide with electrodes so as to scatter the electron emission material occurs to a conspicuous degree. In other words, since the coil of the heater that constitutes the electrodes is arranged sideways so as to be perpendicular to the tube axis of the glass tube, the ions collide with a major portion of the coil. Therefore, ion sputtering occurs to a conspicuous degree. If ion sputtering occurs to a conspicuous degree over an entirety of the coil, the electron emission material is exhausted during discharge, and it is thus impossible to carry out any stable arc discharge over a long period of time. This results in a problem of a reduced service life of the electrodes.
Further, since the electrodes are stretched with tension at the heater, a problem has arisen that after use over a long period of time, they tend to become disconnected.
Thus, the electrodes have a short service life, so that another problem arises insofar that the discharge lamp itself has a shortened service life.
Moreover, since the heater extends perpendicularly to the tube axis, a problem has arisen that a diameter of the tube cannot be reduced.
Further, although a discharge lamp of cold-cathode type, which can be reduced in tube diameter, has a longer service life, it suffers from a large drop in voltage of a cathode, thus resulting in poor efficiency.
The present invention solves these problems and has an object to provide a discharge lamp with a short tube diameter, that is of a higher level of efficiency and longer in terms of service life, an electrode for use in the discharge lamp, a method for manufacturing the discharge lamp electrode, and a lighting system.